Tires is where most of it comes from. If you have an independent suspension, you have more.
I run solid axles. My trailer axle is 3" diameter, my tires are 31's. My cruiser axle is 5" diameter and I have 33's on it.
if you do the math, both vehicles have 14" of clearance not counting the differential on my cruiser. I look at it this way, if I can drive over it, the trailer will clear. For me, that means there's no benefit to independent unless I changed something.
My question to him would be, how hard core are the rocks your driving through? If he wants to crawl with it, he's going to want 35's on the trailer and a skid on the tongue.
My M-100 trailer saw Rubicon and Dusy. My tire size was a bit over 34"